According to BlueLetterBible the outline of Biblical useage is:
"a legion, a body of soldiers whose number differed at different times, and in the time of Augustus seems to have consisted of 6826 men (i.e. 6100 foot soldiers, and 726 horsemen)"
According to Vine's Expository Dictionary of New Testament Words:
Legion:
Otherwise spelled legeon, "a legion," occurs in Mat 26:53, of angels; in Mar 5:9, 15, and Luk 8:30, of demons. Among the Romans a "legion" was primarily a chosen (lego, "to choose") body of soldiers divided into ten cohorts, and numbering from 4,200 to 6,000 men (Gk. speira, see BAND). In the time of our Lord it formed a complete army of infantry and cavalry, of upwards of 5,000 men. The "legions" were not brought into Judaea till the outbreak of the Jewish war (A.D. 66), as they were previously employed in the frontier provinces of the Empire. Accordingly in its NT use the word has its other and more general significance "of a large number."
http://www.blbclassic.org/lang/lexicon/lexicon.cfm?Strongs=G3003&t=KJV

Matthew 26:53 says "Thinkest thou that I cannot now pray to my Father, and he shall presently give me more than twelve legions of angels?" It doesn't say that is how many angels there are. It just says that the Father will give him 12 legions. There may be 10,000 legions of angels. What people miss is that it is an analogy that He is the Son of God and that He is in command. That's the point of His statement.

According to an internet search, a legion can range from 3,000 to 6,000 in number. It's not a fixed number, which varies the answer.
Fixed answers would include questions like, "How many eggs are in a dozen?" (always 12) and "How many inches are in a yard." (always 36)
Questions with varied correct answers might be, "How many children go to church on Sunday?" or "How much sugar will make the coffee too sweet?" or "How many men/angels are in a legion?"

Myriad, derived from the Greek μυριάς (murias) meaning 10,000 or an innumerable number. See Hebrews 12:22 and Jude 1:14; an innumerable number or 10,000 times 10,000 (myriads on angels). Check out a number of differing Bible versions pertaining to the following books: Luke 12:1, Acts 19:19, Revelation 5:11 and Revelation 9:16. Myriads could even be considered a non-quantifiable number just by itself. How many angels are there in heaven? Far more than we can imagine and that does not include the ones bound in Hell.